Low-temperature oxidation of fine UO2 powders

Globally the most used nuclear fuel material is uranium(IV) oxide (UO2) which is produced by powder metallurgical processing. Powders are first compacted into cylindrical pellets, and are subsequently sintered at high temperature to increase their density. The UO2 nuclear fuel cycle is also inseparable from wet-chemical routes: uranium is leached from ore bodies and is further purified via, for example, ion exchange or solvent extraction techniques. Additionally, used nuclear fuel can be reprocessed to separate the elements which contain fissile nuclides (U, Pu) from the fission products. Such reprocessing involves the dissolution of used fuel. Ultimately, the compounds produced via wet-chemical routes −irrespective of their nature, “fresh” or reprocessed− are converted into fine oxide powders.UO2 powder shows a tendency to react with oxygen, especially if the grains have a high specific surface area. Uncontrolled uptake of oxygen is usually undesirable in the fuel production stage, and in very fine UO2 powders exposure to normal air at room temperature may even lead to a pyrophoric reaction. Passivation treatments are required to decrease the reactivity of such UO2 powders. Additionally, if oxidation can proceed continuously the higher oxide U3O8 is formed, and this crystallographic transformation is associated with a volume increase of about 36%. Such a volume increase can result in rupture of storage containers or vessels when not accounted for. Evidently, the reactivity of UO2 towards oxygen may pose a risk. Despite continued research in this domain the oxidation process is still not completely understood, especially in fine powders and at low temperatures (< 100 °C).In this PhD thesis the reaction between UO2 and oxygen has been investigated under conditions related to nuclear fuel production and to storage of UO2 powders and pellets. The research focused on assessing the crystal structure of the different phases which are formed, and also evaluated the mechanisms and kinetics involved in the oxidation reaction. Results of this study allow a better understanding of the issues related to UO2 oxidation, and can be applied to improve the handling of UO2 powders and to evaluate the effects of storage conditions.The crystal structure of UO2, seen as the reference state throughout the research, was first investigated. A discrepancy was found with reported values of the unit cell lattice parameter. Therefore, high-quality UO2 samples were prepared which allowed accurate measurement of the lattice parameter via X-ray diffraction. An improved, more precise value was reported which replaces the existing “generally accepted” value dating from the 1950s.An important intermediate uranium oxide is U3O7. It is the main precursor for U3O8 formation at temperatures below 350 °C. However, the crystal structure of U3O7 remains subject to debate. Polycrystalline U3O7 powders were therefore prepared and investigated using X-ray and selected-area electron diffraction techniques. A comprehensive analysis revealed that the crystal structure consists of fluorite-type ordered uranium and oxygen atoms as in UO2, with excess anions grouped in so-called cuboctahedral oxygen clusters and forming a long-range ordered defect structure.At temperatures below 100 °C the oxidation reaction is not well understood. To shed more light on this temperature domain, fine UO2 powders were oxidized at temperatures ranging from 40 °C to 250 °C and with a different oxygen content in the gas supply. The oxidation process was followed in-situ by simultaneous thermal analysis and the oxidized powders were subsequently analyzed using X-ray diffraction and high-resolution transmission electron microscopy. The initial oxidation reaction is characterized as a rapid chemisorption reaction, corresponding with formation of an amorphous phase on the surface of the grains. As the extent of the oxidation reaction increases different kinetic regimes are distinguished, and separate higher oxides form as nanosized domains throughout the grains.